1. Field of the Invention
This invention relates to print registration and more particularly relates to image registration with a single optical sensor.
2. Description of the Related Art
Printing systems such as duplex image printing systems and color image printing systems often register two or more separate images on one or two sides of a page. The page may be paper, a printed form, a continuous feed printed form, or a carton. Registration involves aligning each image in a desired position relative to each other image or images. Images are mis-registered when one or more images are not aligned as desired relative to the other images.
For example, a duplex printing system prints a first image on the anterior side of a page and a second image on the posterior side of the page. The first image and the second image are considered to be registered if they are aligned relative to each other so that the first image and the second image occupy equivalent areas on the anterior and posterior sides of the paper. When printing color images, the color separated images are deemed to be properly registered when two or more of the color separated images are aligned relative to each other on one side of the page.
Current printing systems often employ registration marks to align the two or more images. FIG. 1 depicts a registration mark system 100 of the current practice. A page 105 is imprinted with a first registration mark 110. In a certain embodiment, the first registration mark 110 is part of an image. The first registration mark 110 may also be preprinted on a single sheet form, preprinted on a continuous feed form, or be a physical characteristic of the page 105. In addition, the first registration mark 110 may be printed on the anterior side of the page 105 and a second registration mark (not shown) may be printed on the posterior side (not shown) of the page 105. Alternately, a color registration mark 105 may be printed for each color separated image of a color image printed on one side of the page 105.
FIG. 2 illustrates a registration detection device 200 of the current practice. The registration detection device 200 includes two optical sensors 205. A paper transport module (not shown) moves the page 105 with the first registration mark 110 of a first image past the first optical sensor 205a and the second registration mark (not shown) of a second image past the second optical sensor 205b. When the first registration mark 110 is in the same position relative to the first optical sensor 205a as the position of the second registration mark relative to the second optical sensor 205b, the first image and the second image are considered to be registered.
In Operation, the first optical sensor 205a detects the first registration mark 110, and the second optical sensor 205b detects the second registration mark. A logic module (not shown) measures the time interval between the detection of the first registration mark 110 and the second registration mark. If the time interval is smaller than a specified limit, the first image and the second image are considered to be registered. If the time interval is larger than the specified limit, the first image and the second image are considered to be mis-registered.
It is a drawback of such a system that the registration detection device 200 must include at least two optical sensors to register the images on the page 105. In addition, the registration detection device 200 requires that the logic module have sufficient sophistication to measure the time interval between the each instance when each optical sensor 205 detects a registration mark 110. Two optical sensors and the sophisticated logic increase the cost of the registration detection device 200.
What is needed is a process, apparatus, and system that detects image registration with a single optical sensor. Beneficially, such a process, apparatus, and system would reduce the cost of a registration detection device.